So, there are a few things that get suggested a lot. Cards that reward you for having Curses, cards that are worth negative VP or give Curses on play, Reactions that deflect the attack, "Veto"-Reactions that keep you from getting the effect of a card you played... Most of these are ideas that don't work well in their basic form, but in some way or another they have nonetheless made their way into Dominion. Quest and a few Events and Landmarks from Empires reward you for having or even buying Curses, and Enchantress is as close to a "Veto" card as it'll get, I think. I did Minister to emulate "negative VP" cards, too.
So what about deflecting attacks? I never tried myself at making this concept non-horrible, so I felt, I got to write my master thesis, and fast. Why not give this a go instead? So, here goes a first concept for a Reaction that deflects attacks. Well, in fact it doesn't do that, but it attacks back harder when attacked.
My original plan was to have it play itself again, but that meant doubling the coin value. Halving the coin value per default seemed too bad to bother with. As is, the second sentence is a bit clunky, but "to make them do it again" could be understood to include players with only 3 cards left. I also tried "Lose your Revenge token to do this twice, otherwise do it once:" at the start instead, but that seemed weird because you don't "do" anything.
Please not that you can reveal an Assassin and put it on your deck after that. Revenge is a dish best served cold, they say.
Edit: I forgot, I also considered calling it "Retaliation Token" instead. What's better?